I was looking to make a dust shoe for my "HY-3040 Column Engraver" CNC and I started building one of the designs I found on here where you attach some brushes to the side of a piece of scrap with a hole for the dust and another for the spindle. The brushes I ordered turned out to be so stiff that they did not really give much when contacting the workpiece and I didn't want to put the extra load on the Z-axis-nor did I want to order different, more expensive brushes.
Prior to starting to make a permanent dust show, I stood next to the CNC with my dust hose manually next to router bit and that worked pretty well so I thought about how I could do the same thing in a more permanent manner.
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Step 1: Cut Scrap to Fit Your Spindle and Dust
Using only the top piece without a skirt resulted in mediocre dust collection because the suction was centered several inches from the cutting. By building an airbox to redirect the suction so it is over the spindle collet, the air velocity around the bit is so great that 98% or more of the chips and dust are pulled in before they can fly away.
I started with an upper layer that has one circle sized for my spindle (80mm) and another circle for a dust collection fitting (4"). I used a quick disconnect fitting that had a tapered opening to ensure a good fit. Make sure the dust collection fitting will clear your CNC, I had to recut my original top piece as the middle piece because my holes were too close together which caused an interference.
The middle layer is just there to to increase the flow area, if you have the clearance and a high volume dust collector ( I used a modified 2hp Harbor Freight unit), I highly recommend something like this to minimize pressure drop. In my cad program, I just drew tangent lines between the two circles from the upper layer. I then glued the upper and middle layers together and drilled holes for 1/4" rare earth magnets.
The lower layer has an opening for the spindle on one side and the other side is hollowed out as much as the strength of material allows to minimize pressure drop. I was using MDF so I left about 0.2" of material. I rounded off any sharp edges for better airflow and then drilled holes for 1/4" magnets and glued them in place-make sure that you don't accidentally glue one in backwards!
Step 2: In Action
Here it is in action. I clamped the shoe so the collet is level with the bottom of the bottom layer. Removing the bottom layer gives me just enough access to change bits or collets.
In the attached video you can see the chips start to fly away but then get sucked back in.